The Numbers Behind Life Expectancy

Can a Cuban baby born today expect to outlive an American baby? That’s one contention of filmmaker Michael Moore in his indictment of the U.S. health-care system, “Sicko.” CNN, in a report that aimed to check Mr. Moore’s facts, reported otherwise. Mr. Moore has fired back about this and other factual disputes, and CNN has issued a rebuttal.

The movie poster for ‘Sicko’

The life-expectancy dispute revolves around a very small difference in numbers. Mr. Moore’s Web site cites United Nations stats showing (page 283) that Cubans’ life expectancy is 77.6 years, or one-tenth of a year more than in the U.S. CNN, in turn, cites a World Health Organization report it says is more up-to-date, showing Americans have life expectancy of 77.9 years, compared with 77.2 years for Cubans. (A spokesman and a publicist for Lions Gate Films, which is distributing “Sicko,” didn’t respond to my requests for comment. A spokeswoman for CNN directed me to the network’s written response to Mr. Moore.)

To understand the dispute, it’s worth reviewing how life expectancy is calculated, why the underlying numbers can be problematic and whether it makes sense to compare countries by this measure.

Demographers usually define life expectancy as the number of additional years a person of a certain age can expect, on average, to live. In the press, life-expectancy numbers, such as those that feature in the Moore-CNN debate, are life expectancy at birth. All life-expectancy numbers, though, derive from what’s called a life table, which shows, for each age x, the percentage of those people who reach age x who then survive to age x+1. For example, a 70-year-old American in 2003 had a 97.7% chance of living another year, according to the Centers for Disease Control. The survival percentages come from dividing the number of people of each age who die by the total number of people of that age; and subtracting that percentage from 100%. (For more details, see page 8 of this life-expectancy report from the CDC.)

Life expectancy comes from following a hypothetical group of, say, 100,000 people from birth down the life table; calculating how many are likely to die at each age; and calculating their average (as in mean) age at death. The downside to this calculation scheme is that it assumes that the group of people born in 2007 and surviving to 2077 will experience the same death rate at age 70 as do 70-year-olds today.

“We’re not following a real cohort of people,” Robert Anderson, chief of the CDC’s mortality-statistics branch, told me. Dr. Anderson added that the usual presumption is that medicine will improve over time, meaning current life-expectancy stats are understated. However, unforeseen events — including the introduction of new diseases — can create the opposite effect. (Life-expectancy rates fell in many sub-Saharan Africa countries after the wide spread of AIDS.)

Life-expectancy numbers also are only as good as the underlying data, which can be confounded by faulty self-reporting and inconsistent data-collection methods. Americans born before 1933 may not have birth certificates, as registration wasn’t yet mandatory in some states, Dr. Anderson said. Also, older Americans who aren’t sure of their age tend to round up on Census forms — perhaps because reaching a certain age is a point of pride, he said. What results is so-called “age heaping” at older ages ending in 5 or 0 — and therefore flawed death rates. The CDC attempts to get around that for ages over 85 by using data from Medicare, which requires some proof of age. “That’s not perfect, either, but it’s more likely to more accurately reflect the death rates,” Dr. Anderson said.

University of Pennsylvania sociologist Sam Preston said the ages reported by older black Americans are especially rife with errors because they were less likely to have received birth certificates.

Francois Pelletier, chief of the U.N.’s mortality section, also pointed out that while the U.S. and Cuba have reliable vital-registration systems for counting deaths, many less-developed countries don’t, adding to the uncertainty. The life expectancy in the U.S. and Cuba is “almost identical, statistically speaking,” Mr. Pelletier said.

A bigger question is whether life expectancy is a reliable indicator of the overall quality and accessibility of a country’s health-care system. Penn’s Prof. Preston noted that the life expectancy calculated for Americans born today reflects the relatively high death rate among older American men who smoked regularly — a phenomenon that can be mitigated, but not explained, by doctors and hospitals. The same goes for rates of death by car accident, homicides and suicides and underlying societal or economic conditions such as poverty and hunger that can’t be attributed to the health-care system but do affect health. “To suggest that differences in the medical-care system fully explain differences in life expectancy is a big reach — not that it isn’t a big factor,” Dr. Anderson said.

What do you think? How much does it matter whether Americans live longer than Cubans? Are life-expectancy rates accurately forecasting how long people live? What’s the best measure for comparing two health-care systems?

About The Numbers

The Wall Street Journal examines numbers in the news, business and politics. Some numbers are flat-out wrong or biased, while others are valid and help us make informed decisions. We tell the stories behind the stats in occasional updates on this blog.